Electromagnetic logging devices for determining the electrical properties of subsurface formations are well known. For example, U.S. Pat. Nos. 4,209,747 (Huchital) and 4,511,843 (Thoraval) describe electromagnetic propagation tools used to measure the dielectric constant and the conductivity of formations surrounding the borehole of an oil well. These tools launch an electromagnetic signal into the formation and detect the signal after it has propagated through a known distance to one or more receivers where the phase and/or amplitude of the signal is measured. Since formations of differing dielectric and conductivity properties cause the phase and amplitude of the propagating signal to be modified by various amounts, the dielectric and conductivity properties of the formation may be deduced.
U.S. Pat. No. 4,651,101 (Barber, Chandler, and Hunka) describes an induction logging tool with metallic support used to measure the conductivity of the formation surrounding the borehole of an oil well. Copending U.S. patent application No. 115,503 to B. Clark, M. Luling, J. Jundt, and M. Ross describes an electromagnetic logging device which provides two or more radial depths of investigation. The electromagnetic logging devices described in these patents have the following features in common; several antennae disposed along the length of an elongated tube, one or more antennae acting as transmitters of electromagnetic radiation, and two or more antennae acting as receivers or detectors of electromagnetic radiation.
An important practical consideration in the logging of the electrical properties of geological formations is that of providing a method for testing and calibrating the electromagnetic logging device immediately before logging, and immediately after logging the oil well. Such testing/calibration is best performed on the oil well rig floor to assure the proper, contemporaneous operation of the tool. This calibration is required to determine whether the transmitting and receiving antennae are operating correctly, to determine the threshold signal detectable at each receiver, and to check the electronic circuitry used to power the transmitters and the electronics used to measure the phases and/or the amplitudes of the signals at the receivers. In order to calibrate electromagnetic logging tools such as described in the above patents, it is necessary to induce an electromagnetic field with a known phase and known amplitude at each receiving antenna. The expected phases and amplitudes can be compared to the measured phases and amplitudes, and the differences can be used to correct any subsequent (or previously obtained) readings If the readings are too different from the expected values, then the tool is not functioning correctly, and would not be run in the well.
The present invention provides a method and apparatus for testing/calibrating electromagnetic logging devices at the wellsite immediately before and after logging the well. The simplest and most often used previous method for testing electromagnetic tools has been to operate the tools with the antennae directed into air. The resulting phase and amplitudes measured by the receivers were then compared to those values expected for an electromagnetic field radiated in air. This could be performed on the rig floor, but the close proximity of metal objects would interfere with the radiated field thereby introducing an uncertainly in the test/calibration Another disadvantage with this method has been that the dielectric constant of air is unity and its conductivity is zero, so air is not representative of the conditions encountered in any subsurface geological formations.
Typical values for the dielectric constant of geological formations lie in the range of 5 to 50, and typical values for the conductivity of formations lie in the range of 0.001 to 10 mhos/meter. Normally, the phases and amplitudes measured with an air test lie outside the normal range of values expected when logging typical formations. Furthermore, the electromagnetic wave travels from the transmitting antennas to the receiving antennas with very little attenuation, so the automatic gain control functions of the tool's electronics were not adequately tested with the previous air technique.
Previously a technique for the calibration of an Electromagnetic Propagation Tool has been proposed and employed for the calibration of a logging tool known as the EPT tool. This tool comprises a pad which is pressed against the borehole wall and includes a transmitter and two receivers for detecting phase shift and attenuation The operating frequency of the EPT tool is 1.1 GHz so that the spacing between the transmitter and the receivers and between the two receivers is quite small when compared to the spacings of electromagnetic propagation tools whose operating frequency is in the range of from 0.1 MHz to 10 MHz. This prior technique calls for the installation of a conductive barrier between the two receivers which serves to introduce a specified phase shift and attenuation therebetween. A variety of barriers of varying heights and thicknesses may then be utilized to simulate the effects of a range of phase shifts and attenuations that might be expected from a variety of geological formations.
The above described technique is effective for logging tools such as the EPT tool which have high operating frequencies and short spacings between the transmitter and the receivers (on the order of inches). Interference to the transmitted electromagnetic fields by neighboring metallic objects can be avoided by tools having short spacings merely by assuring that the tool being calibrated is no closer to such objects than a few feet. Tools that operate at lower frequencies with much longer transmitter - receiver spacings, however, are much more likely to be subject to interference when located on the floor of a drilling rig since nearby metallic equipment, such as the drilling derrick or the floor itself effects the operation of the tool. Additionally, as previously mentioned, it would be desirable to have a calibration technique that is performed at values representing those encountered in the borehole rather than at values available from an air calibration which are outside of the range of those of the geological formations.